Conventional rubber-modified styrenic resins (HIPS) have had the problems that they have a low impact resistance and are inferior in a surface gloss of the molded articles and in a hue obtained by toning with a dye or a pigment as compared with those of an ABS resin. Accordingly, a method has recently been proposed in which the particle diameter of rubber is decreased and the impact resistance is supplemented by silicon oil for the purpose of improving a balance between the impact resistance and the gloss, whereby it is brought close to the characteristic of an ABS resin. However, an HIPS composition obtained by such method has not necessarily been satisfactory in terms of a coloring property while a balance between the impact resistance and the gloss is improved.
In order to improve this defect, methods are proposed in Japanese Patent Publication No. Hei 3-62723 and Japanese Patent Publication No. Hei 1-34453, in which the particle diameter of an aromatic monovinyl polymer included in a dispersed rubber particle is made fine and uniform to thereby improve the coloring property and the mechanical properties. These methods are characterized by using specific polybutadiene and making the particle diameter of an aromatic monovinyl polymer included in a dispersed rubber particle fine to 0.13 micron or less and so uniform that 90% of the number thereof falls in the range of .+-.0.04 micron.
The coloring property is certainly improved by these methods. On the other hand, the mechanical strength, particularly tensile elongation and drop impact strength are notably reduced. A reduction in these mechanical strength has brought about the problem that the injection-molded article is liable to be broken when it is released from a mold or dropped, and therefore the improvement has been desired.